Lift onions when most of the tops have flopped over and turned yellow. Then cure them in airy shade until the necks are tight and the skins feel papery. Only cured bulbs go into storage. Pungent, long-day varieties kept cool and dry can feed a household into March, while sweet types are for eating in the first weeks.
When do you lift onions in Illinois, and how do you cure them?
How do you know the bulbs are ready to lift?
The plant tells you. Once a bulb has finished swelling, the neck softens and the tops fall over on their own. When most of the row has gone down and the leaves are yellowing, stop watering and let the soil dry. Pull on a dry day, loosen the soil with a fork first, and handle the bulbs gently, because a bruise is where rot gets in.
Day length sets the whole schedule. Chicago sits at 41.99 degrees north and gets 15 h 04 of daylight on the longest day of the year. Long-day onions wait for 14 to 16 hours before they start bulbing, so northern Illinois gives them what they need. Short-day onions begin bulbing at about 12 hours of daylight, according to Clemson Cooperative Extension. Here they bulb early and stay small. A gardener in the far south of the state is at a lower latitude than Chicago, so check your own latitude before you assume the same calendar. Your county extension office can tell you what your local bulbing window looks like.
How do you cure onions after lifting?
Curing dries the neck and outer skins so they seal the bulb. Spread the onions in a single layer, or hang them in small bunches, somewhere shaded with moving air. A covered porch, a barn aisle or a garage with the door open all work. Direct sun can scald the bulbs, and rain sends you back to square one. Illinois summers are humid, so a spell of muggy weather slows the job. A box fan set on low helps.
Skip the washing. Brush off dry soil with your hand and leave the tops on while the bulbs dry. Curing is done when the neck is thin, tight and completely dry, and the outer skins rustle. Then trim the roots, and cut the tops to a short stub. A bulb with a thick, wet neck that will not tighten is not a keeper. Use it first. Damaged bulbs, and ones with rot creeping in at the neck, belong in the kitchen pile too, and What Eats Onion in Illinois, and When covers the pests that leave those wounds.
Keeping them, and for how long
What temperature and humidity do cured onions need?
Clemson Cooperative Extension’s Home & Garden Information Center puts it in one line: “Ideal conditions are between 45 °F and 55 °F, and 50 to 60 percent humidity.” The full factsheet is at hgic.clemson.edu. The number to write down is 45 °F to 55 °F with 50 to 60 percent humidity. Tape it inside a cabinet door if you have to.
Humidity is where gardeners go wrong. Onions want dry air. Carrots, beets and potatoes want the opposite, a cold, damp cellar. One space cannot serve both crops well, and an onion in damp air will sprout or rot at the neck. If you grow both, plan on two storage spots.
Keep the onions in single layers or loose containers so air moves around them. Mesh bags, slatted crates and old nylon stockings with a knot between each bulb are the usual choices. Sealed plastic bags and closed buckets trap moisture and ruin cured bulbs.
How long do onions keep in storage?
Any duration you hear is a ceiling under ideal conditions. A bulb with a nick in it, a slightly damp neck, or a warm shelf will go sooner. For a well-grown, fully cured pungent onion held in the range above, the realistic best case in the northern states is that it lasts into March. Plan on the early weeks of winter being the safest stretch, and check the pile every couple of weeks, pulling out anything soft or sprouting. One bad bulb can spoil its neighbors.
Why do sweet onions not keep?
University of Missouri Extension (G6355, Planting, growing and harvesting onions) says it directly: “As a rule, ‘sweet’ onions do not store as well as the more pungent types.” The reason is chemical. The bite of an onion comes from volatile sulfur compounds, and those same compounds help protect the bulb while it sits in storage. A sweet variety carries fewer of them. That makes it better raw and a poor candidate for the winter shelf.
Day length lines up with this. Short-day types run sweeter, and long-day types keep longer. A southern garden gives you onions to eat soon, and a northern one gives you onions that can feed you well into the late winter. Never count on a sweet onion to last. Eat those first, and store the pungent ones. The Best Onion Varieties for Illinois sorts out which type is which.
Where in the house that actually is
Which room in an Illinois house is closest to the range?
The target is cool, dry, dim and ventilated. Most houses do not have a room that fits exactly, so you are picking the best compromise.
- Refrigerator: colder than the range onions tolerate, and the crisper drawer is humid. Fine for a cut half for a few days, wrong for the harvest.
- Heated living space: kitchens, pantries beside the stove and rooms over a furnace run too warm, and the bulbs sprout early.
- Garage or shed: swings with the weather. Muggy in summer, and in a hard Illinois cold snap it can freeze the bulbs.
- Basement: often cool, but frequently damp, especially in summer. Good only if it stays dry.
For most households the closest match is an unheated interior room, such as a spare bedroom on the cool side of the house, a closet against an inside wall, or a dry corner of the basement far from the furnace and laundry. It does not need to be elegant. It needs to be steady.
What should you check with a thermometer?
Do not guess at a spot. Put a thermometer with a humidity readout where the onions will sit and watch it for a week before you move the harvest in. Skipping that step is how a basement that feels cool turns out to sit in damp air, and a month later you are sorting soft bulbs.
Read it morning and evening, and again after a rainy stretch. If the temperature runs inside 45 °F to 55 °F and humidity stays at 50 to 60 percent, you are set. If humidity runs high, a small dehumidifier or a fan can pull it down. If a spot runs warm, move to another spot rather than hoping. Garages and porches are worth testing as well, and a fall reading will not match a January one, so check again when the weather turns.
Common questions
Can I store sweet onions through the winter?
Not reliably. Sweet onions have fewer of the sulfur compounds that protect a bulb in storage, so they sprout and soften sooner. Cure them like the rest, use them first, and keep your pungent varieties for the long haul.
Should I wash onions before storing them?
No. Moisture on the skins invites rot. Let the soil dry, brush it off by hand, and store the bulbs dry and clean-looking rather than scrubbed.
Can onions share a cellar with carrots and potatoes?
Poorly. Onions need dry air at 50 to 60 percent humidity, while roots keep best in cold, damp conditions. Put the onions in a separate, drier spot, or the roots and the onions will each suffer.
What if the tops never fall over by themselves?
Look at the necks. If the foliage is yellowing and the bulbs are full size but the tops are still standing, bend them over gently and let the soil dry. Then lift once the leaves are mostly brown. A bulb with a thick, green neck keeps poorly, so use it first. If you are unsure of your timing, ask your county extension office. For the big picture of growing them here, start with Onions in Illinois.